The use of chelating or sequestering agents in acidizing technology allows for the design of fluid systems offering benefits over conventional acidizing treatments (see H. Nasr. El-Din et. al., 2014 SPE 168145; Reyes et. al., 2013 SPE 165142; Reyes et. al., 2013 SPE 164380; H. Nasr. El-Din et. al., 2011 SPE 128091; M. Parkinson, et. al., 2010 SPE 128043; and W. W. Frenier et. al., 2001 SPE 71696). Specifically, the agents give rise to improved complexation of cations and stabilize the minerals formed within a formation matrix as a result of an acidizing treatment. For example, when chelating agents along with HF acid are utilized in sandstone matrix acidizing treatments, cations formed by the dissolution of minerals such as silica, feldspar, and feldspar are held in solution via cation-chelation, reducing precipitation of salts formed from the cations, resulting in improved matrix permeability (U.S. Pat. No. 8,316,941). For this reason, the solubility profile (e.g., pH for dissolution) of the chelant as well as the nature of the chelant that is introduced into the treatment fluid (e.g., as the acid, sodium, or potassium salt) is important.
Aminopolycarboxylate chelants akin to ethylenediaminetetracetic acid (EDTA) are one class of compounds useful in the oil and gas industry as anti-scalants, iron-sequestrants, and standalone matrix stimulation fluids. These organic compounds function as reactivity modifiers of multivalent cations. For instance, chelating reagents such as N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), glutamic acid N, N-diacetic acid (GLDA), methyl-glycinediacetic acid (MGDA), and hydroxyiminodisuccinic acid (HIDS) chelate or complex the cations formed during the dissolution of formation minerals resulting in improved solubility profiles (Dorota Kolodynska (2011). “Chelating Agents of a New Generation as an Alternative to Conventional Chelators for Heavy Metal Ions Removal from Different Waste Waters (Chapter 17)” Expanding Issues in Desalination, Prof. Robert Y. Ning (Ed.)).
Some challenges attach to the use of the foregoing chelating agents. First, commercially available chelants are typically supplied as their sodium or potassium salts. In order to remove the sodium or potassium ions and generate a neutral chelating agent, the pH must be lowered by introduction of hydrochloric acid for effective carbonate acidizing. This also has added consequences when having to employ hydrofluoric acid (HF) in sandstone acidizing. Post-synthesis modification, such as ion exchange process, of the sodium salt of a chelant, for instance, can be implemented to produce the ammonium salt of the chelant which is then free of cation (Na+ or K+) or in low concentration (see US20130281329).
Second, the post-synthesis acidification mentioned above liberates large amounts of unwanted cations into oil well treatment fluids. Specifically, sodium (Na+) and potassium (K+) cations strongly associate with fluoride and fluorosilicates, causing the rapid precipitation of metallated-fluorides or metallated-silicates, and thereby mitigating the efficacy of the acidizing treatment (see R. D. Gdanski, 1994 SPE 27404).